Ellen Page, left, and Olivia Thirlby star in the coming-of-age comedy Juno, about a pregnant 16-year-old trying to figure out what's best for herself and her baby.

Ellen Page, left, plays a pregnant 16-year-old, and Michael Cera the unprepared father of the child in the comedy Juno.

Early in “Juno,” our heroine – 16, grunge-cute and hyperarticulate – flips open her hamburger phone and declares, “I’m calling to procure a hasty abortion.”

There are probably a thousand ways a movie could have conveyed that information, but this smart comedy has a knack for picking the funniest, freshest wisecrack available. Which is why the final entry in Hollywood’s 2007 unplanned pregnancy derby isn’t merely the top of its division, smoking “Knocked Up,” “Waitress” and “Bella.” It’s one of the year’s best films in any category.

The premise could have spawned a dire Hallmark movie. When her first sexual experiment leaves her pregnant, a Minnesota high-schooler decides against that hasty abortion, and begins looking for the ideal adoptive parents for her child. But instead of a treacle-dipped sermonette about responsibility and redemption, “Juno” delivers uproarious laughs, fully fleshed personalities, honest uplift and tender moments when the throat goes dry and the eyes grow moist.

Much of the credit goes to the deservedly acclaimed script by former Twin Cities scribe Diablo Cody, whose blogs-to-riches story seems destined to culminate in a spotlight solo at the next Academy Awards.

The simple but intricate story scrambles our responses and covers so much ground, with such zest, that you regret it ending. If Lynda Barry and Dorothy Parker collaborated, the result could hardly have warmer affection for its fallible characters or sharper strokes of whiplash wit.

The 20-year-old Canadian actress Ellen Page, who resembles a baby Christina Ricci, grabs the title role in both tiny fists and runs away with it. Juno is reflexively sarcastic, but Page’s uncannily expressive performance reveals the uncertainty behind the girl’s pose.

When her best pal urges her to check the prospective parents’ ads in the local PennySaver, Page ticks off her preferences: “graphic designer, mid-thirties, with a cool Asian girlfriend who totally rocks the bass – but I don’t want to be too particular.” The film is shrewd about the gap between what people think they want and what they actually need, and as the lesson plays out, Page’s work transforms and deepens along with Juno’s understanding. Comedy performances don’t often win Oscar attention, but Page, a versatile actress who brought feral anger to the rape-revenge thriller “Hard Candy,” deserves to be an exception.

One character that grows convincingly is rare enough, but “Juno” has a half-dozen.

J.K. Simmons (“Spider-Man’s” gruff J. Jonah Jameson) and Allison Janney (“The West Wing”) play Juno’s dad, Mac, and stepmom, Bren, a decent couple more bewildered by their daughter’s pregnancy than scandalized. Juno’s spiky relationship with Bren blossoms into a surprisingly warm friendship as they outgrow the kid/parent dynamic and begin to relate woman to woman.

Michael Cera (“Superbad”) plays the child’s father, nerdy track star Paulie Bleeker, with his trademark “who, me” look of male adolescent sexual embarrassment, but gives it more feeling than ever before. He matures to the point that Mac, who grumbled that he’d “punch that Bleeker kid in the wiener the next time I see him,” does something very different instead.

The key to the movie is Juno’s evolving relationship with infertile yuppies Mark and Vanessa Loring (Jason Bateman and Jennifer Garner). When Juno and Mac enter their inhospitably glossy McMansion in upscale Glacial Valley, the Lorings appear to be off-the-rack stereotypes. Vanessa is prim and repressed while Mark, who introduces himself with, “I’m the husband,” seems to have the emotional depth of a sitcom sidekick. But “Juno’s” characters are endlessly surprising. In a scene of infinite tenderness, Vanessa tells the little stranger in Juno’s swollen belly, “I can’t wait to meet you.”

Mark, a wryly funny onetime rocker now grinding out jingles, has the hipster credentials Juno wanted, but he’s not done being a child himself. Juno, who bonds with Mark over horror movies and pop music, introduces a new set of stresses into the couple’s relationship, resulting in the most gasp-worthy plot twist of the year. Garner and Bateman have never been this good before.

Cody’s work is ideally served by director Jason Reitman, who established himself as a talent to reckon with in the flashy satire “Thank You for Smoking.” He operates here in a down-to-earth mode ideal for “Juno’s” story. Without that naturalism, the artifice of its verbal fireworks could be off-putting.

“Juno” has its flaws, to be sure. The characters sometimes speak with a single voice – even the neighborhood druggist talks like the wisenheimer teens – and most of the folks onscreen can’t clear their throats without inventing a kooky metaphor. Juno’s hard-rocking taste in music is at odds with the soft and fuzzy soundtrack. But such minor failings scarcely matter. “Juno” invites you into a world you won’t want to leave.